Defect or anomaly detection in structures is often important in determining maintenance intervals, or for determining whether structures require repair or replacement. Non-destructive detection of structural anomalies may be desired, and the ability to perform timely and effective examination of objects may not necessarily be made easier when the objects are large, may be remotely located relative to large population centers, and may be subject to harsh geographic or climatic conditions.
By way of example, the inspection of pipelines is a task of some interest and economic importance to the owners and users of pipelines, and in particular as it pertains to pipelines for carrying hydrocarbon gases and oils, although pipelines for transporting other fluids and slurries are also known. A typical pipeline for carrying gas, oil or water may run for many miles between pumping stations. The pipeline may be exposed to the weather, and that weather may include a corrosive atmosphere, be it a salt spray environment or some other. The pipeline may run through regions of greater or lesser humidity, and the weather may vary from season to season between extremes of heat and cold. In some places the pipeline may be carried above ground on spaced supports. In others it may be buried, or partially buried. In locations in which the pipeline is buried, the surrounding stratum may have a high or low moisture content, and may be alkaline or acidic. The fluid, or slurry to be carried in the pipeline may itself not be benign, but may be of an aggressive nature, and may be abrasive or corrosive, or both. The material flowing in the pipeline may be under significant pressure, perhaps in the thousands of psi., and may be at an elevated temperature, possibly in the range of 80-100 C. This environment may effect not only the life of the pipeline and the nature of the defects that may be expected to be found in a section of pipe over time, but also the tools used for monitoring and maintaining the pipeline. Stress cracking and stress corrosion may occur or be hastened by movement related to temperature change, earthquakes or tremors, ground settling, vibration from fluid movement, and pressure changes in the medium during operation.
Even assuming that a region of interest has been identified, and, if necessary, that there has been digging to expose that region to permit access for inspection, the pipewall to be inspected may not be fully accessible. That is, the pipewall may be protected by an external covering or coating. Removal of the covering or coating may not be desirable or practical, and, once the inspection is complete, replacement of the removed covering or coating may not necessarily be an easy or inexpensive task. Further still, pipeline inspection may occur under less than optimal conditions. For example, inspection under wintry conditions may be particularly problematic. The inspector may be encumbered in bulky clothing. The use of writing instruments may be difficult. The inspector may be up to his or her waist in snow, on uneven terrain, with uncertain footing. The wind may be uncomfortably raw or brisk. At other times of year, the inspector may be beset by black flies. In short, pipeline inspection may be a miserable task, and the accuracy and reliability of the results may reflect this.
The foregoing discussion has been made in the context of pipelines. However, it might also be made in the context of other large structures—be they the hulls of ships, railroad car bodies, seagoing drilling or production platforms, rail road tracks (and railroad wheels, in situ), bridge structures, and so on.